Are there any plans on supporting IThumbnailProvider methods currently?
Yes, it's planned for a future version.
Is that still the case? What about installing the 32bit distribution of Adobe Reader?
Adobe Reader is still 32-bit only. (Adobe have been quite useless
at supporting 64-bit users
The old (pre-Vista) style of thumbnail provider runs in-process so it has to be the same "bitness" as the process requesting the thumbnails. Same reason that Adobe Reader thumbnails don't appear in Explorer on 64-bit machines.
A 32-to-64 bridge could be written to make it work (like the one I wrote to allow viewing PDF files in 64-bit Opus before I discovered how to fix Adobe's preview handler for 64-bit) but it's quite a lot of work just to make one thumbnail format work again when the fault is really on Adobe's side. It's something I might look into in the future if time permits, though.